The steam flow path in a steam turbine generally is formed by a stationary casing and a rotor. A number of stationary vanes may be attached to the casing in a circumferential array and extend inwardly into the steam flow path. Similarly, a number of rotating blades or buckets may be attached to the rotor in a circumferential array and extend outwardly into the steam flow path. The stationary vanes and the rotating buckets may be arranged in alternating rows such that a row of stationary vanes and the immediately downstream row of rotating buckets form a turbine stage. The stationary vanes serve to direct the flow of steam such that it enters the downstream row of rotating buckets at an efficient angle. The airfoil portion of each rotating bucket extracts energy from the flow of steam so as to develop the power necessary to drive the rotor and a load attached thereto.
As the flow of steam passes through the steam turbine, the pressure drops through each succeeding stage until a desired discharge pressure is achieved. As such, the properties of the flow of steam such as temperature, pressure, velocity, moisture content, and the like may vary from stage to stage as the flow of steam expands through the flow path. Consequently, each row of buckets may have an airfoil shape that is optimized by the steam conditions associated with that row. Other configurations of steam turbines also may be known.
It is generally recognized that the performance of a steam turbine may be greatly influenced by the design and the performance of the later stage buckets operating at the reduced steam pressures. Ideally, the last stage buckets should efficiently use the expansion of the flow of steam down to the desired turbine exhaust pressure while minimizing the kinetic energy of the flow of steam leaving this last stage. Improving efficiency at the later stage buckets thus should improve overall efficiency of the steam turbine.
There is therefore a desire for improved steam turbine designs and related performance, particularly for the buckets of the last or the later stage of a low pressure steam turbine and the like. Such an improved turbine bucket design should improve overall steam turbine efficiency and performance while limiting flow separation, wake losses, and other types of flow path instabilities impacting on the flow of steam therethrough. Such improvements also may be applicable to any type of axial flow turbine including a gas turbine.